In order to address environmental issues and to improve fuel efficiency, hybrid cars (hereinafter, vehicles) equipped with both a motor and an engine have been on the market in recent years. These vehicles regenerate electricity during braking by using a vehicle braking system which employs electrically hydraulic control instead of mechanically hydraulic control which is conventionally used. The vehicle braking system, however, may not operate if a fault occurs in the battery.
To solve this problem, there is provided an auxiliary power supply which supplies electricity to the vehicle braking system at the time of battery failure in Patent Document 1, for example. FIG. 5 is a block circuit diagram of such a power supply device. The power supply device includes capacitor unit 101, which is composed, for example, of a plurality of electric double layer capacitors with large capacity and connected to each other as electric storage elements. Capacitor unit 101 is connected to charging circuit 103 and discharge circuit 105, which control the charging and discharging of capacitor unit 101, respectively. Charging circuit 103 and discharge circuit 105 are controlled by microcomputer 107. Microcomputer 107 is connected to voltage detector 109 which detects a failure of the battery. Voltage detector 109 is connected to FET switch 111 which supplies electricity stored in capacitor unit 101 to electronic control unit 117 at an emergency. Electric storage device 113 having this structure is connected between battery 115 and electronic control unit 117, and is started and stopped by ignition switch 119.
Electronic control unit 117 is a vehicle braking system. To ensure safety of the vehicle, electronic control unit 117 has to continue to operate even if battery 115 fails. Therefore, upon detection of the failure of battery 115 by voltage detector 109, microcomputer 107 turns on FET switch 111 to supply the electricity stored in capacitor unit 101 to electronic control unit 117. When the use of the vehicle is finished, microcomputer 107 controls discharge circuit 105 to discharge the electricity stored in capacitor unit 101, thereby suppressing a decrease in the property of capacitor unit 101.
Microcomputer 107 determines the property deterioration of capacitor unit 101 when ignition switch 119 is turned on to start the vehicle power supply device, which is required to be highly reliable. More specifically, the property deterioration is determined as follows. First, when the power supply device is started to charge capacitor unit 101, the internal resistance of capacitor unit 101 is measured either from the voltage measured at the start of the charging or from the voltage measured when the charging is interrupted. Next, the internal capacitance of capacitor unit 101 is calculated from the voltage change rate per unit time of capacitor unit 101 during the charge process. Microcomputer 107 then compares these values with the respective normal values, thereby determining the property deterioration of capacitor unit 101.
When started to charge capacitor unit 101, electric storage device 113 can determine the property deterioration of capacitor unit 101 and can also address the failure of battery 115. When, however, used as an emergency power supply device for instantaneous power failure, electric storage device 113 cannot fully determine the property deterioration because of the following reason. An emergency power supply device always has to have electricity for an instantaneous power failure that could occur at any time. Therefore, the property deterioration can be determined only when the emergency power supply device is initially started up, not while in use.